1. Field of the Invention
This invention relates to a process for the catalytic reductive methylation of an acid hydrazide wherein the latter is pretreated to eliminate catalyst poisoning contaminants.
2. Description of the Prior Art
Mono methyl hydrazine (MMH) and unsymmetrical dimethyl hydrazine (UDMH), due to the ability of these compounds to provide hypergolic ignition upon contact with a strong oxidizer, represent important liquid propellant fuels, particularly for space vehicles. Although there are several routes available for the commercial production of these compounds, all, however, involve problems of one type or another.
For example, the most expedient process for producing UDMH consists of hydrogenating nitrosodimethylamine in turn obtained by nitrosating dimethylamine. This process, however, has been banned for all practical purposes because of the extremely potent carcinogenic nature of the nitrosodimethylamine intermediate. A currently practiced process for producing MMH is in accordance with a modified Raschig synthesis involving the in situ generation of mono-chloramine and subsequent reaction with monomethyl amine. The principal disadvantage of this process is that it is excessively energy intensive.
A hitherto proposed process commercially applicable for producing UDMH as well as MMH, and the process to which the present invention relates, consists of the reductive alkylation of an acid hydrazide with formaldehyde in the presence of hydrogen and a suitable hydrogenation catalyst and subsequent base hydrolysis or hydrozinolysis of the resultant alkylate.
The most effective catalyst for this purpose is either platinum or palladium, preferably the latter supported on carbon. In accordance with the process, optimum yield of UDMH, and likewise MMH, is in the order of 80-85%. In order to realize such a yield, however, it is necessary to take the necessary precautions to avoid the poisoning of the catalyst during the course of the reaction.
Free hydrazine present in the reaction mixture, even in the amount of one percent or less, can have a very deleterious effect on catalyst activity. Although the preferred procedure for preparing the acid hydrazide intermediate involves the use of a substantial deficiency of the hydrazine, an unreacted amount thereof in the order mentioned is invariably present. A suitable way of eliminating free hydrazine is to adjust the pH of the hydrazine solution to neutral or slightly on the acid side with acetic acid or like acids or anhydrides. However, these acids contribute to overall cost, are corrosive and consume caustic in the hydrolysis reaction. It is, accordingly, the object of this invention to provide an alternative way for eliminating the free hydrazine present in an acid hydrazine prepared in the manner indicated without resort to the use of an acid as aforesaid.